1. Field
The invention relates to a polarization mask for producing linearly polarized light having a predetermined polarization direction from incident light. The polarization mask is useful in the exposure of polarization-sensitive layers, for the preparation of such components, and for their preferred use. The term xe2x80x9cproducingxe2x80x9d is to be understood for the purposes of this description as denoting both the actual production of linearly polarized light from unpolarized light and changing the polarization direction of incident light which has already been linearly polarized.
2. Description
Various processes and materials have recently become known in which anisotropic material properties are induced by irradiation with linearly polarized light. The anisotropic propertyxe2x80x94that is, the preferred direction of the material in microscopically small zonesxe2x80x94can be varied by location of the polarized light: In addition to optical anisotropy it is more particularly the steric anisotropy which gives these materials their interest for industry. Liquid crystals in contact with a polymer material are oriented in accordance with the preferred direction thereof by the steric anisotropy of such material. These photostructurable polymers are therefore eminently suitable as orientation layers for liquid crystals, it being possible for the orientation direction to be varied in the micrometer range.
U.S. Pat. No. 4,974,941, the contents of which are herein incorporated by reference, describes a process based on a guest-host system wherein a preferred direction is induced by irradiation with linearly polarized light of an appropriate wavelength by the cis-trans-isomerization of dyes. Liquid crystals in contact with a surface thus irradiated are oriented in accordance with this preferred direction. This orientation process is reversiblexe2x80x94that is, by further irradiation of the layer with light of a second polarization direction, the orientation direction already written in can be turned again. This reorientation process can be repeated as often as required and is therefore particularly interesting in connection with re-alignable optical memories.
U.S. Pat. No. 5,389,698, the contents of which are herein incorporated by reference, discloses photostructurable orientation layers in which, in contrast to the reversible orientation process hereinbefore described, an irreversible anisotropic polymer network is built up. The optical and orienting properties induced in the network during the exposure with linearly polarized light are photostable and so cannot be further re-oriented by further exposures. These photo-oriented polymer networks (PPN) can therefore be used wherever stable, structured or unstructured LC orientation layers or optical elements such as non-absorptive color filters, linear and cholesteric polarization filters, optical delay layers or the like are needed.
As a rule, in the exposure of a photostructurable orientation layer at least two different polarization states of the light have to be used to write in a definite pattern. Various exposure processes for impressing polarization patterns into photostructurable layers are known at present. All the known processes have the disadvantage that the total information cannot be transmitted in parallel fashion but must be transmitted seriallyxe2x80x94that is, in more than a single exposure step. The known processes are therefore complex, costly, and time-consuming.
For instance, a scanner can be used to apply the information to the photostructurable layer in dot form. In this case the polarization direction can be varied from dot to dot. However, in the transfer of high-information-content patterns only a very short exposure time is available for each dot if the total exposure time for the pattern is not to exceed acceptable limits. Consequently, the energy necessary for the orientation must be applied to a small region of the layer in a short time, so that the heat loadability of the photostructurable material is severely stressed.
Another possibility, described in U.S. Pat. No. 5,389,698, the contents of which are herein incorporated by reference, is for the layer to be irradiated with polarized light through a mask. This enables all the zones or regions of a layer which have the same orientation direction to be exposed simultaneously. Many different orientation directions can therefore be written into a layer by the use of further masks. An exposure step is necessary in this process for each orientation direction, the transmission direction of the polarization filter having to be adjusted and the mask having to be changed and positioned each time. Positioning the mask is a particularly time-consuming operation.
Since a particular transmission direction of the polarizer is associated with each mask in this stepwise exposure process, the polarizer itself can be part of the mask. A polarizing mask of this kind can be produced in various ways by known technologies, for example, by laminating a film on to a polarization film. A polarizing mask of this kind can be produced in various ways by known technologies, for example, by laminating a film on to a polarization film. A polarizing mask of this kind could be improved if all the necessary polarization directions could be integrated in a single mask so that a complete polarization pattern could be transferred to a photostructurable layer in a single exposure step. A polarization mask of this kind for the preparation of LC orientation layers is mentioned in EP-A-632 311, but no indication is given about how such a polarization mask might be produced. In fact, the stretching process conventional in the preparation of polarization films automatically leads to a consistent polarization direction over large areas.
U.S. Pat. No. 5,327,285, the contents of which are herein incorporated by reference, discloses a process for producing polymerizers having two polarization directions which differ zonewise in the micrometer range. The process is based on the technology for producing polarizer films. The polarizing properties of two polarizer films are extinguished zonewise by chemical or mechanical treatment and stuck together accurately at a 90xc2x0 offset from one another. However, due to the stringent requirements for positioning the two films and sticking them together the differently polarizing zones cannot be made as small as required. Also, parallax errors caused by the stacking of the relatively thick polarization films on one another limit the number of possible polarization directions to two.
It is the intent of the present invention to provide a polarization mask that enables production of linearly polarized light with zonewise differing polarizing directions.
The subject invention provides a polarization mask for producing linearly polarized light having a predetermined polarization direction from incident light and having a light input side and light output side. The polarization mask comprises a plurality of zones which are limited from each other on the light output side and which have at least intermittently different polarization directions.